The present invention relates to certain polymeric materials, and compositions containing them which are charge transport materials. The invention also relates to processes for making these polymers and their use in devices such as electroreprographic devices and electroluminescent devices.
Polymers of the invention may be used in the field of electroreprography. Electroreprography is any process in which an image is reproduced by means of electricity and incident radiation, usually electromagnetic radiation, more usually visible light. Electroreprography includes the technology of electrophotography which encompasses photocopying and laser printing technologies. Typically, in both a photocopier and a laser printer, a photo-conductive member is first charged in the dark (e.g. by applying a high voltage via a Corona discharge). Then a latent electrostatic image in charge is produced by partial exposure of the charged photo-conductive member (e.g. a drum or belt) to radiation (e.g. light). The radiation neutralises the charge in the exposed regions. The light source can either be reflected light from an illuminated image (photocopying) or from a laser which scans the photo-conductive member usually under instruction from a computer (laser printing). Once a latent image has been produced in charge, it is developed with toner, the toner is transferred onto a substrate (e.g. paper) and then fixed thereto (e.g. by heat) so that a visible image is obtained.
The photo-conductive member typically comprises a photo-conductor (e.g. an organic photo-conductor [xe2x80x9cOPCxe2x80x9d]) which must perform two different functions: generate a charge on exposure to the incident radiation; and transport the photo-generated charge to the surface. The unexposed regions of the photo-conductive member will retain their charge and form the latent image. It is usual to use different materials for each of these two processes and develop materials which are separately optimised for their ability to generate photo-induced charge (charge generating materials or xe2x80x9cCGMsxe2x80x9d) or their ability to transport charge (charge transport materials or xe2x80x9cCTMsxe2x80x9d). The photo-conductor can be constructed as a single layer or from a plurality of layers, for example from at least one charge generating layer (xe2x80x9cCGLxe2x80x9d) comprising the CGM and at least one separate charge transport layer (xe2x80x9cCTLxe2x80x9d) comprising the CTM.
An ideal photoconductor would be one where the material charges rapidly to a high value in the dark, retains the charge in the dark (i.e. exhibits no dark decay) and shows rapid total discharge on exposure to low-intensity illumination. The time taken for the charge-discharge cycle of a photo-conductor limits the maximum speed at which the latent image can be generated. Photo-conductive materials with improved electrical properties may allow faster printing and copying and/or higher quality copies and/or longer component life.
The applicant has discovered means to improve the charge transport properties of certain polymers based on triaryl amine repeat unit(s).
PCT/GB98/03685 is a co-pending patent application which describes novel CTM polymers of repeat unit of Formula D 
where Y1 is N, P, S, As and/or Se and Ar1, Ar2 and Ar3 are aromatic groups. These polymers are prepared by the addition of an end capping reagent to control the molecular weight of the final polymer and hence its desirable properties as a CTM.
The disclosure of this co-pending application is incorporated herein by reference; its definitions and description should be used in connection with the present invention except as modified herein.
The procedure of the co-pending application produces attractive charge transfer agents. However we have now found that they may be further improved and indeed similar-materials which are not end capped may be improved by isolating an appropriate molecular weight fraction from them. We have found that medium molecular weight. polymers are generally superior to lower and higher molecular weight polymers derived from the same starting materials; lower molecular weight polymers are less effective and crystallise more readily and are thus not durable; higher molecular weight polymers are durable but less effective. By means of this invention a fraction may be selected which is of good performance and is sufficiently durable to cover the required life of components of electroreprographic devices whilst optimising its charge transport effectiveness.
According to one aspect of the present invention there is provided a process which comprises isolating, from a polymer comprising repeating units which are individually of Formula X 
in which
Y is P or N,
Ar1 and Ar2 are bivalent aromatic groups,
Ar3 is a monovalent aromatic group, and
the units of Formula X may be the same or different,
a molecular weight fraction which is a charge transport material.
According to a another aspect of the present invention there is provided a process in which an improved charge transport material is produced by isolating, from a first charge transport material which is a polymer comprising repeating units which are individually of Formula X 
in which
Y, Ar1, Ar2 and Ar3 are as previously defined; and
the units X may be the same or different,
a molecular weight fraction of improved charge transport properties.
Ar1 Ar2 and Ar3 preferably comprise benzenoid rings which are optionally fused with other, preferably benzenoid, rings and/or substituted.
The polymer may be a block copolymer comprising such repeat units.
Trivalent repeat units of Formula D may be included to permit a degree of chain branching if desired.
The molecular weight fraction produced by the process according to the present invention may be of Formula 1:
AXmBxe2x80x83xe2x80x83Formula 1
in which
each X is a unit of Formula X as defined above and may be the same or different;
A and B are chain terminating groups, for example, hydrogen, chlorine, bromine or iodine, or other leaving groups used in a polymerisation process by which the polymer is made, or end capping groups, and
m is the average number of X units per molecule of the fraction.
The value of m is suitably 4 to 50, preferably 4 to 30, more preferably 4 to 25 for example 4 to 15 and especially 5 to 13 or 6 to 14. Generally it is desired that m is at least 5 in all of the above ranges.
The polydispersity of the fraction is suitably 1.1 to 4, preferably 1.1 to 3 and more preferably 1.2 to 2.5. Suitably the fraction is substantially free from molecules having 3 or fewer or 50 or more repeat units.
The aryl groups may be mono- or poly-cyclic and the rings are preferably benzene rings substituted with, for example, one or more C1-40-alkyl groups in order to increase their solubility in for example tetrahydrofuran (THF) for processing purposes. They may have fused ring groups, naphthyl groups or covalently linked benzene rings for example biphenyl residues but are preferably benzene rings which are each substituted with alkyl groups providing a total of one to eight aliphatic carbon atoms per benzene ring.
Preferably the groups A and B are inert to coupling with further molecules of the polymer so as to reduce the likelihood of further growth of molecules to undesired sizes. Thus it is preferred that they should not be halide atoms.
Molecular weight fractions produced by the present invention may be isolated from polymers prepared without the addition of a separate end capping reagent to control the molecular weight of the polymer.
Isolation of the molecular weight fraction may be by filtration technologies, chromatographic techniques, osmotic methods, and/or solid extraction using a suitable solvent, for example Soxhlet extraction.
In a preferred form of the invention (which may be used together with one or more of the above techniques) the process comprises a step of partially precipitating from a solution of the polymer in a suitable solvent, a molecular weight fraction thereof. It may be separated by dissolving the polymer in the solvent, precipitating a least soluble (highest molecular weight) fraction and then recovering a fraction of greater solubility having good CTM properties, for example in electrophotographic applications, from the remaining solution. The least soluble fraction may be usable as a CTM in non-electrophotographic applications. Examples of suitable solvents are dichloro- and trichloro-ethane, dichloro- and trichloro-ethylene, dichlorobenzene, toluene, dioxane and, more preferably, THF. It will be appreciated that it is not necessary to use any other isolation procedure in conjunction with this preferred form of the invention.
If there are undesired low molecular weight molecules present, they may be allowed to remain in the solution when a desired fraction is recovered. The fractions may be recovered by
(a) cooling the solution and collecting successive fractions in the course of cooling, or
(b) evaporating solvent from a solution of the polymer and collecting successive fractions or
(c) more conveniently, differential precipitation, that is, by the addition of a precipitant to a solution of the polymer and collecting successive fractions at increasing concentrations of the precipitant, in which a precipitant is suitably a liquid which is miscible with the solvent but in which the polymer is sparingly soluble.
The precipitant may be selected from n-octane, heptane, n-hexane, cyclohexane, methylpentane, n-butanol, n-propanol, 2-propanol, ethanol, methanol, acetone, mixed alkanes such as petroleum ether (pet ether) with boiling range 60-80xc2x0 C., methyl-t-butyl ether (MTBE), high boiling mixed alkanes such as those available under the trade name Isopar (e.g. Isopar G with a boiling point of about 160xc2x0 C.); and/or mixtures thereof. The chosen precipitant has a boiling point (at ambient conditions) preferably from about 50xc2x0 C. to about 200xc2x0 C., more preferably from about 60xc2x0 C. to about 170xc2x0 C. It is preferably a lower alcohol, for example a C1-6-alkanol, such as ethanol or propanol or, more preferably, methanol, or a lower alkane or cycloalkane, preferably a C4-10-alkane or cycloalkane, or acetone. An especially preferred precipitant is methanol.
The molecular weight fraction collected by the isolating means preferably has an average number of repeat units (xe2x80x98mxe2x80x99 number) of from about 4 to about 15, more preferably from about 5 to about 13.
The isolated molecular weight fraction preferably has a much narrower molecular weight distribution than the crude polymer.
According to a further aspect of the present invention the polymerisation may be performed in the presence of a controlling means to control molecular weight of the polymeric material during polymerisation (and before isolation) to provide a polymer suitable for use as a charge transport material, more preferably which is electroreprographically effective, most preferably with an average number of repeat units xe2x80x98mxe2x80x99 of between about 4 and about 25. The means for controlling MW may be any of those described in co-pending British patent Application No 9914164.1 (the priority of which is claimed for this application) and/or those described in co-pending application PCT/GB98/03685.
However, even if the crude polymer is already effective for use as a CTM after polymerisation and without isolation, because a means to control molecular weight has been used to prepare the polymer, the isolation means according to the aforementioned aspects of the present invention may be used to isolate a fraction having further improved properties as a charge transport material. Alternatively, the isolation means may be used to convert crude polymer which is not especially fit for this purpose into an effective charge transport material. Thus, the combination of the two methods of control of molecular weight during polymerisation and isolation of molecular weight fraction from the crude polymer after polymerisation can be used flexibly to achieve cost effective production of polymer with the required performance as a charge transport material. The polymeric materials of the present invention are preferably obtained by polymerisation controlled by addition of at least one end capping reagent in an amount sufficient to reduce substantially further growth of the polymer chain. The end capped polymers of the invention can be produced more cheaply and with a better control over their resultant properties (such as their molecular weight and polydispersity) due to the end capping. Furthermore the chemical nature of the end cap can be selected to control aspects of the polymerisation and hence properties of the resultant polymer. For example carrier mobility, polymer compatibility, electronic configuration [e.g. frontier orbital (FO) energy levels] and/or solubility may be affected by substitution (if used) and/or molecular weight (e.g. mobility can be shown to increase with polymer molecular weight).
For optimum electroreprographic performance preferably the polymers of the invention are substantially free of chlorine, more preferably substantially free of chloro, bromo and iodo, containing species.
Polymers of the present invention preferably have a weight average molecular weight (Mw) from about 1000 to about 13000 daltons.
In Formulae 2 and 3 hereinafter a suitable terminal group such as hydrogen or another substituent which is inert to coupling under the conditions of polymerisation, eg an akyl or aryl group, can be selected to control aspects of the polymerisation and Ar1, Ar2 and Ar3 are each an optionally substituted aromatic group which may be a mononuclear aromatic group or a polynuclear aromatic group. A mononuclear aromatic group has only one aromatic ring, for example phenyl or phenylene. A polynuclear aromatic group has two or more aromatic rings which may be fused (for example napthyl or naphthylene), individually covalently linked (for example biphenyl) and/or a combination of both fused and individually linked aromatic rings. Preferably each Ar1, Ar2 and Ar3 is an aromatic group which is substantially conjugated over substantially the whole group.
Polymers of the present invention may be made by a polymerisation process which is controlled to limit further growth of the polymer chain. If the end capping reagent is generated in situ in excess (e.g. at the step when it is desired to terminate polymerisation) further growth of the polymer chain (and/or polymer network if the is polymer is branched and/or cross-linked) can be substantially inhibited (e.g. substantially quenched). The end capper adds terminal group(s) to the polymer chain which are substantially incapable under the conditions of polymerisation of undergoing coupling (e.g. with other polymer precursor and/or other parts on the polymer chain). The terminal group(s) end cap the polymer chain and act to substantially reduce the possibility of (preferably stop) further polymerisation by blocking sites at which the polymer chain could otherwise grow under the conditions of the polymerisation. Preferably in the polymers of the present invention from about 60% to substantially all of the polymerisation sites are blocked by at least one terminal substituent. More preferably (in one option) substantially all such sites are blocked. In another more preferable option from about 60% to about 90% of these sites are blocked.
Optional features of polymer fractions of the present invention, which may further distinguish them from known polymers, are that they can, if desired: comprise terminal group(s) other than a group selected from H, halo, hydroxy, glycidyl ether, acrylate ester, methacrylate ester, ethenyl, ethynyl, vinylbenzoxyl, maleimide, nadimide, triflurovinyl ether, a cyclobutene, a group forming part of a cyclobutene group, and trialkylsiloxy; and/or be other than copolymer(s) which consist of triarylamine repeat unit(s) and C4-7alicyclic repeat unit(s) optionally containing heteroatom(s); and/or be substantially undoped; and/or substantially polydisperse and/or other than of formula: 
wherein n is from 7 to 11.
Polymer fractions of the invention preferably comprise at least 4, and more preferably at least 6, repeat units of Formulae X or 1 or Formulae 2 or 3, hereinafter.
Preferred polymer fractions of the present invention comprise molecules represented by Formula 2: 
wherein
Ar1, Ar2, Ar3 and Y represent, independently in each case, those atom(s) and/or group(s) as described herein;
n represents an integer from 3 to about 500;
R1 and R2 represent, independently, a terminal group as described herein.
In Formulae X and/or 2, Ar1, Ar2, and Ar3 are preferably, each independently, optionally substituted aromatic groups, more preferably having optionally substituted heterocyclic and/or benzenoid rings. Most preferably the optionally substituted aromatic groups Ar1 and Ar2 are, or form part of, bivalent C6-40-hydrocarbyl groups, preferably phenylene and/or naphthylene groups. Any substituents are preferably C1-15-alkyl groups.
Polymeric material of the present invention may comprise molecules represented by Formula 3
wherein
R1 and R2 represent terminal groups which are unreactive groups, that is are substantially incapable of undergoing chain extension under the conditions of polymerisation;
a and b represent, independently in each case, 0 or an integer from 1 to 4;
c represents, independently in each case, 0 to 5;
n represents an integer from 4 to about 200; and
R4, R5 and R6 represent, independently in each case, optionally substituted C1-15alkyl groups and/or one or more other substituents.
The terminal groups (which may be attached to the repeat units of Formula X, denoted by A and B in Formula 1 and denoted by R1 and R2 in Formulae 2 and 3 are preferably unreactive groups, that is are substantially incapable of undergoing chain extension or cross-linking under the conditions of polymerisation. They are more preferably independently selected from hydrogen or C1-40-hydrocarbyl groups, preferably selected from C1-30-alkyl, C6-36-aryl and C7-36-aralkyl groups, any of which may optionally be substituted. Especially preferred terminal groups comprise C6-36-aryl groups optionally substituted with at least one C1-4-alkyl, C1-4-alkoxy, or amino group (itself optionally N-substituted by at least one C1-4-alkyl). In particular the terminal group may be selected from phenyl, optionally substituted with at least one methyl, 2-methylprop-2-yl, methoxy, ethoxy, trifluoromethyl and/or diethylamino group.
The novel polymer fractions of the present invention are of use as CTMs in electroreprographic devices. However such polymers may have many other uses which may rely on the same, similar and/or different properties to those required for electroreprography.
For example the polymers of the present invention may be generally relevant for use in (and/or in combination with) any application and/or device which requires the use of polymeric conductors, polymeric photo-conductors, organic photo-conductors (OPCs), electroluminescent (EL) materials, polymeric materials which exhibit substantial conjugation over the polymer and/or polymeric semiconductors. Preferred polymeric semiconductors have hole mobilities greater than 0.01 cm2/volt.sec. This minimum mobility is either that of the pure polymeric material, or of an admixture of the polymeric material with one or more other polymeric or monomeric materials having different electrical and/or physical properties. Preferably the polymers of the present invention also exhibit some or all of the following other useful properties: high carrier mobility, compatibility with binders, improved solubility, high durability and/or high resistivity undoped.
The polymers of the invention may be used in at least one of the devices and/or for at least one of the applications described in co-pending PCT application GB98/03685.
Certain of these applications may require tuning of the properties of the polymers of the invention. For example polymeric CTMs of the invention when optimised for use with organic light emitting materials (OLEMs) preferably may have a higher molecular weight and/or different mobilities than are optimal for electroreprography.
Furthermore the compositions and/or specific polymers used for each application may be different. For example it is desirable that an electroreprographic polymeric CTM is compatible with the binder polymers (such as polycarbonates) used to make a CTL. By comparison a polymeric CTM for use in an OLEM may be formulated without many other (or even no other) ingredients to make a film of substantially pure CTM. Thus each of these CTM polymers may require different physical properties.
The invention is illustrated by the following Examples. Unless indicated to the contrary, or clearly different from the context, all references herein and in the following examples and experiments to percentages refer to the percentage by mass of ingredient to total mass of the composition to which the ingredient is to be added or of which it is a part.
The number average molecular weights (Mn) quoted in the Examples herein were determined by gel permeation chromatography (Waters 150CV) calibrated against polystyrene standards. Samples were run in tetrahydofuran (hereinafter xe2x80x9cTHFxe2x80x9d) using two xe2x80x9cPolymer Labs. Mixed Dxe2x80x9d gel columns at a rate of 1 ml/min. A value for Mn was determined from the GPC spectrum, and from the Mn value, an approximate average degree of polymerisation (xe2x89xa1m as defined herein) was calculated by subtracting the mass of the terminal groups and dividing by the molecular weight of the repeat unit.
The electrical properties given in the Examples herein were obtained in the following test methods.
A number of electrophotographic photoreceptors were prepared as described below.
Titanyloxy phthalocyanine (TiOPc) type IV (15.0 g) was dispersed into a 5% w/w solution of polyvinyl butyral (PVB) in n-butyl acetate (75.0 g) using a high shear mixer. A further quantity of n-butyl acetate (20.0 g) was added to the dispersion to reduce its viscosity. The resulting slurry was charged to an Eiger Mini 50 Motormill (supplied by Eiger Torrance Ltd.) containing a charge (34 ml) of 0.6 to 0.8 mm zirconia beads. The mill was operated at 3,000 rpm for 50 minutes. PVB solution (25.0 g, 5% w/w in n-butyl acetate) was added to the millbase and milling was continued for a further 10 minutes. The millbase was discharged into a receiving vessel and PVB solution (61.5 g) was added to the mill and circulated for 5 minutes. The solution was then discharged into the millbase which was stirred throughout to prevent pigment agglomeration and n-butyl acetate (349.0 g) was flushed through the bead mill and out into the stirred dispersion to yield a CGL coating formulation of PVB (1.48%), TiOPc (2.75%) and n-butyl acetate (95.77%).
The dispersion was coated onto aluminised Melinex film using a K#2 bar and K Control coater model 202 (supplied by RK Print-Coat Industries Ltd.). The coating was dried for 5 minutes at 100xc2x0 C. to produce a CGL which was approximately 0.4 xcexcm thick.
Preparation of a Charge Transport Layer (CTL) of the Invention
A formulation comprising a CTM of the invention was prepared using an amount of a CTM and (optionally) another CTM as specified in the Examples. If not otherwise specified herein 0.5 g of CTM was used (equivalent to 25% CTM in the CTL) in the following preparation. The polymeric CTM and polycarbonate resin (1.5 g of the PCZ available commercially from Esprit Chemical Co. under the trade designation TS 2020) were dissolved in toluene (7.1 g). This solution was coated on top of the CGL made as described above, using a 150 xcexcm wet film depositing bar and K Control coater. The coating was dried for 90 minutes at 100xc2x0 C. to give a CTL which was approximately 25 xcexcm thick.
A semi-transparent metal electrode was applied to the top of a section of the film by vacuum deposition. The CTL thickness was measured using an Elcometer E 300 device. A small portion of the CGL and CTL (prepared as described above) close to the top electrode, was removed with a suitable solvent to reveal the bottom electrode. The electrodes were connected to a power supply and a digitising oscilloscope.
The time of flight (TOF) technique is a transient photoconductivity experiment well known to those skilled in the art. A field was applied across the sample via the electrodes and a sheet of charge carriers (holes) was photogenerated at one side of the film. The charge carriers drifted through the film under the influence of the field creating a current which was detected using a current amplifier connected to the oscilloscope. When the carriers reached the counter electrode, the current was observed to decrease and the transit-time across the film could thereby be determined from the transit waveform. The measurement was repeated with a range of different applied voltages.
The drift mobility of carriers (xcexc) was calculated for each applied field (=V/L) using the equation:
xcexc=L2Vxe2x88x921ttr31 1,
where L is the device thickness, V is the applied voltage and ttr is the transit time. Mobility at a field strength of 160 kV/cm was determined by interpolation of the mobility vs F plot. A field strength of 160 kV/cm is typical of what may be present in a working photoreceptor. A high value of mobility is desirable because it indicates that the photoreceptor will discharge rapidly on exposure to light.
A number of electrophotographic photoreceptors were prepared as in the TOF technique described herein. A photoreceptor test piece of approximately 5xc3x9710 cm was cut out from the coated aluminised Melinex. The test piece was then fixed to a bare aluminium drum (used as the substrate for an OPC), 30 mm in diameter. Two small areas of coating were removed from the edge of the test piece using a suitable solvent. The test piece was then electrically connected to the drum using a suitable conductive paint. The drum was then mounted in a QEA PDT 2000 device (available commercially from Quality Engineering Associates Inc. Burlington Mass. 01803 USA) and was grounded via the contact in the QEA instrument. The QEA PDT 2000 was fitted with a 780 nm band pass filter. A track with a consistent xe2x88x92800 V charge of at least 10 mm length was selected using the charge scanner. Once the track had been selected the PIDC was measured in the known manner. The energy required to discharge the photoreceptor to xe2x85x9th of its original potential i.e. from xe2x88x92800 V to xe2x88x92100 V (Exe2x85x9e)and the residual potential on the photoreceptor after the highest energy exposure (xcx9c3 xcexcJcmxe2x88x922) (Vr) were recorded. Low values for Exe2x85x9e and Vr are desirable in a photoreceptor as they indicate efficient discharge of the device on exposure to light.
OPC devices can be prepared in a similar manner to the method described above using different CGLs, binders and/or additives in combination with other CTMs and CGMs, for example as described in co-pending application PCT/GB98/03685 (e.g. in the tables thereof.